Two registers: in everyday speech 套叠 means nesting/overlapping (Russian dolls, telescoping cups). In medicine, 肠套叠 (literally 'intestine nest-fold') is the technical term for intussusception, where one section of bowel slides into another.
memory
套 (to sheath/cover) + 叠 (to fold/stack) - picture one thing slipping inside another and stacking up. That image works for both Matryoshka dolls and the medical sense.
